Several instruments used in the analysis of samples, such as biological samples, comprise a light source to illuminate the sample and an optical detector to perform a photometric measurement. In clinical chemistry and some immunochemistry assays, for example, the optical transmission of a liquid sample is measured through a cuvette. The results are used to generate extinction data, which is the ratio between light intensity input and light intensity after transmitting the sample. Optical extinction can be caused either by absorbance or by scattering of the light by the sample. In the case of absorbance, extinction is caused by the selective absorbance of light by a particular substance typically at selected wavelengths. In the case of scattering, extinction is caused by the light being deviated in all directions by particles or drops in the carrier liquid. Scattering is typically independent from the wavelength. Particularly, scattering can be used to measure the concentration of macromolecules or particles in solution. Given a fixed total concentration of one or more species, scattering signal is a direct measure of the weight-averaged molar mass of the solution, which will vary as complexes form or dissociate. Therefore, reagents which form complexes with analytes present in a sample, such as latex beads whose surface has been derivitized with analyte binding groups, are typically used in this type of assays, also called turbidimetric assays.
In this way, the presence and/or concentration of analytes in a sample, which may be indicative of a clinical diagnostic condition, can be determined by measuring response signals.
Both forms of extinction measurements, absorbance and scattering, suffer from possible interferences, i.e., undesired substances in the sample, which may lead to deviations or errors in the measurement.
In scattering assays, extinction is due to scattering caused by a substance of interest in the sample or like in turbidimetric assays by reaction processes between the substance of interest and one or more reagents. The measurement however may be erroneous in the case of, for example, lipemic samples. Lipids suspended in samples are known to also cause scattering and, thus, to interfere with the measurement of the substance of interest.
Another type of interference in scattering assays is an absorbing substance that interferes with measurement of scattering. However as scattering can be observed at any wavelength and the absorbing interference is usually confined to a specific wavelength range, it is usually possible to circumvent the interference by a suitable choice of measurement wavelengths.
An interference in absorbance assays is due to scattering caused by undesired substances such as lipids in lipemic samples, which disturbs the absorbance measurement.
Since a scattering interference is present at all wavelengths, sensitivity to scattering automatically causes disturbance of absorbance assays.